Theoretical studies of molecular structure and vibrational spectra of the asymmetric squaraine [(2-dimethylamino-4-anilino) squaraine]

Theoretical calculations were performed using density functional theory for studying the molecular structure and vibrational spectra of the asymmetric squaraine [(2-dimethylamino-4-anilino) squaraine] (ADTCH3) in the ground state. The influence of intermolecular interactions effects on molecular properties have been considered by calculation performed on (ADTCH3) dimer. The optimized geometric bond lengths and bond angles obtained by using B3LYP/6-31G(d) show a good agreement with the experimental data. As compared to theoretical frequencies of the monomer, the calculated values obtained for (ADTCH3) dimer are in much better agreement with the experiment. All the experimental vibrational bands have been discussed and assigned to normal modes on the basis of our calculations. With respect to the study of Silva et al., some modes have been reassigned in the light of present theoretical results. In virtue of time dependent density functional theories (TD-DFT), the low-lying excited states of (ADTCH3) are determined using 6-31G(d) basis set. The calculations show that using TD-B3LYP/6-31G(d) approach, experimental absorption spectrum has been well reproduced. It has been concluded that the lowest singlet excited state (S1) of the title compound is mainly derived from the (HOMO → LUMO) (π → π∗) electron transition located on oxocarbon ring.